For fish farming, cages made of nets produced from elastic cords are used, as disclosed by publication WO-A-96/13973. The plastic nets made of a synthetic material are generally dimensioned with a cord diameter of, for example, between approximately 4 and 6 mm and relatively small mesh openings of 10 to 30 mm so that even young or small fish cannot escape. With these cages made of plastic nets, there is the risk that when the latter are assembled in open seas, predators such as sharks, seals or sea lions may rip open the cages from the outside with their mouth or will bump against said cages at a relatively high swimming speed and damage them so that the animals to be farmed may swim out in an undesirable manner at these damaged points.